Interference Effects in Nanocrystalline Systems
- PDF / 864,480 Bytes
- 6 Pages / 593.972 x 792 pts Page_size
- 89 Downloads / 239 Views
DUCTION
RECENT studies have shown how interference effects can be observed in the diffraction from polycrystalline systems when scattering domains are sufficiently small and textured.[1–5] Besides loose nanocrystalline powders, where the above conditions can arise from special growth or coalescence phenomena,[6,7] these effects are important also in polycrystalline aggregates as they can influence the overall properties of the system.[3] Models have been proposed to describe the special condition when the diffracted intensity distributions from small and closely oriented domains overlap in Reciprocal Space (RS), thus affecting the observed powder patterns in different ways.[1–5] This is especially visible for low Miller indices peak profiles, corresponding RS points of which are closer to the origin. Further insights have been provided by a few recent studies, where interference effects among the nanosized metallic domains were simulated using atomistic models and the Debye scattering equation (DSE)[8,9] to generate the corresponding powder patterns.[10,11] In all of these cases, however, interference was related to the size and orientation of the domains without the consideration of the role of grain boundaries, which indeed contribute a considerable fraction to nano-scale materials (see, e.g., References 12 through 14). Models and simulations so far have treated nanocrystals as small (perfect) single crystals, without accounting for the boundary region and of the natural tendency of the system to achieve a minimum of energy. In the current study, we provide a preliminary understanding of the possible effects of the grain boundary (GB) on the diffraction patterns from systems made of small crystalline domains. In particular, it is shown that the signal from the GB region can be partly coherent with the bulk of the neighboring crystalline domains.
II.
GENERATION OF THE NANOPOLYCRYSTALLINE MODEL
A nano-polycrystalline cluster made of 50 grains of Cu was built via Constrained Voronoi Tessellation (CVT[15]) in a cubic box of 260.28 A˚ with PBCs (Figure 1). Using this technique, the size and shape of the grains were tuned to obtain a lognormal dispersion of normalized volumes (mean volume 352657 A˚3, standard deviation = 0.35; the diameter of a sphere having the mean volume is ca. 70 A˚). The distribution character was confirmed by the Kolmogorov-Smirnov test at 5 pct significance level. A fcc copper structure (cell parameter a = 3.615 A˚) was placed in the grains, eliminating those atoms on the boundary whose distances pffiffiffiare below 85 pct of the minimum ideal value (3:615= 8).[16] The distribution of misorientation angles agree with the MacKenzie result[17] associated to a randomly textured material. The system was equilibrated at 100 K (173.15 °C) under NPT conditions using molecular dynamics and the Embedded Atom Method potential for Cu.[18,19] The LAMMPS code[20] was employed. Once stationary conditions were reached, a sequence of 100 independent frames was collected each 1 ps. The position of each atom (labeled with the grai
Data Loading...
